Waves in Shallow Water & Tsunamis Flashcards
The longer the wavelength, the _____ (faster/ slower) the wave travels
faster
There is negligible water movement below depth=___ wavelength
1/2
When waves enter water shallower than their wave base, they interact with the _______
seafloor
What happens when waves interact with the seafloor?
- the seafloor prevents water particles from moving in circular orbits –> particles immediately above the seafloor can only move back and forth
- orbits of molecules become stretched out ovals
- this compression, plus friction, slows the forward motion of the wave
For deep-water waves, C is a function of:
For shallow-water waves, C is a function of:
For transitional waves, C is a function of:
wave length
water depth
both water depth and wave length
As water depth decreases, wave speed ___
decreases also
for shallow water waves, wave speed is a function of water depth!
T/F
All waves in shallow water travel at the same speed in the same depth water, regardless of period
true!
As waves enter shallower water and are slowed, does their period change? Does the wavelength change?
Period- no
Wavelength- yes. it decreases
Do each of these increase or decrease when a wave enters shallow water?
- speed (C)
- wavelength (L)
- height (H)
- steepness (H/L)
- speed (C) decreases
- wavelength (L) decreases
- height (H) increases
- steepness (H/L) increases
Explain why wave height and steepness increase in shallow waters
energy must be conserved; kinetic energy is converted to potential energy so height and steepness increase
Explain wave refraction
waves usually approach the shore at an angle= one part of the wave is in shallow water & the rest is in deeper water, refracting (bending) the wave
–> as this process continues to bend the wave, the crests tend to become aligned parallel to the shore
Wave refraction=
result=
= the bending of shallow water waves due to changes in water depth
result= waves normally end up reaching the shore ~parallel to it
surf zone=
the area offshore within which waves are breaking
–> wave steepness increases until the wave becomes unstable and breaks in this surf zone
Waves break in different ways, depending on 3 factors that determine how quickly the wave becomes over-steepened and unstable:
1
2
3
wave period
height
slope of the ocean floor
Spilling wave break=
seafloor almost flat; crest tumbles down the forward face of the wave (foam); break progressively
plunging wave break=
seafloor moderately steep, bottom part of the wave slowed more quickly than the upper part; the crest curls over in front of the wave and plunges downward until it crashes into the trough preceding the wave
collapsing wave break=
seafloor steep; lower part of the wave slowed so rapidly that the leading face collapses before the crest arrives (foam); crest collapses behind with little splash
surging wave break=
very steep shores; waves appear not to break; simply surge up and down with little bubble production (no foam); most of the wave energy is reflected
Why are wave conditions for surfing generally better on the west coast than the east coast of north america?
- fetch: the pacific is bigger than the Atlantic, so bigger waves can develop
- beach slopes are generally steeper on pacific coasts, creating plunging breakers
- onshore winds: winds generally onshore (westerlies), enhancing waves on west coast and weakening them on the east coast
If you’re stuck in a rip current, what should you do?
swim parallel to the beach (don’t try to swim against it)
Explain how rip currents occur
- the water dumped nearshore by breakers must return offshore
- when waves approach at an angle, water is transported along the beach in the direction of the waves
- eventually water encounters an area where is can flow back out more easily
- wave heights are smaller there
- large amounts of water through a narrow corridor –> fast rip currents
_____ are caused by abrupt displacements of ocean water.
Tsunamis
Tsunamis originate from sudden changes in the topography of the ____. Give some examples of what could cause these changes
seafloor
- earthquakes (slippage along underwater faults)
- underwater avalanches
- collapse of large oceanic volcanoes
- underwater volcanic eruptions
- meteorite or asteroid impacts (less frequent)
The majority are caused by vertical displacements of seafloor along faults, which change the volume of the ocean basin and affect the entire water column
A tsunami behaves as a __-water wave everywhere in the ocean
shallow
Tsunamis wave speed is determined by:
They have much ___ (longer/shorter) periods than wind waves
water depth
(because it acts as a shallow water wave)
longer
t/f
tsunamis form a huge breaking wave at the shoreline
FALSE
a tsunami doesn’t break at the shoreline –> its wavelength is so long that even a large increase in wave height does not produce steep, unstable waves
tsunamis move over __km/hr and have heights of __m or less
700km/hr (212m/s)
wave heights of 1m or less –> in deep water, ships can pass unnoticed
Tsunamis are often misnamed ____ ____. Is this accurate?
tidal waves
Not accurate, tsunamis have no relation to tides
They do resemble a sudden, extremely high tide, which is where this name comes from. It’s a strong flood or surge of water that causes the ocean to advance/ retreat dramatically
T/F
The first tsunami surge is usually the largest
false
tsunamis are a series of waves –> alternating series of dramatic surges and withdrawals of water (every 5 to 10min)
the first surge is rarely the largest
The size of a tsunami partly depends on the size of the _____
earthquake
Is the Richter scale linear?
No
An increase of one unit of magnitude (eg 8.1 to 9.1) represents a 10-fold increase in wave amplitude or ~30-fold increase in energy released
